The present invention relates to the detection of pipe load changes on steam turbine casings during turbine assembly and start-up and shutdown transients.
During turbine startup, the reactive loads on the casing vary as steam flow through connected pipes causes the pipes to thermally expand or contract. These load variations can cause the casing to deflect and translate (collectively referred to as “deflections”). Deflection of the turbine casing can result in seal damage as radial rubbing steam seal rubbing increases steam leakage flow and reduces turbine efficiency.
Seals within a steam turbine generally include teeth on the stationary components which interlace with lands on the rotor-bore and bucket covers. The radial gaps between the stationary and rotating sealing features are designed as narrow as possible to minimize steam leakage. Deflection of the casing can result in rotating parts coming into contact with the stationary seals. Contact between rotating parts and stationary seals damages the seals and results in increased steam leakage.
Excessive force on the casing due to pipe connections can occur during the assembly or operation of a turbine. Excessive cold forces on the turbine casing can result if recommended installation procedures are not followed during assembly of the steam turbine. Excessive and varying forces on the turbine casing can develop during turbine startup and shutdown transients due to thermal expansion of the piping system and the steam turbine. The differential expansions between the pipes and casing apply forces, moments and torques on the casing that deflect and translate the casing shells.
Excessive piping loads can also deflect and translate the turbine casing during turbine transient operations. Piping loads during transients, if sufficiently large, can result in a loss of radial clearance control between the rotating parts, e.g., bucket covers, and the stationary seals. A consequence of the loss of radial clearance control is that rubbing may occur between the rotating parts and stationary seals. Radial rubbing will increase steam leakage through the seals. Accordingly, excessive pipe loads may damage the seals between the rotating and stationary parts such that turbine performance is degraded.
Current methods for measuring forces, moments and torques imparted by interconnecting pipes on turbine casings are often inaccurate and expensive to execute. For example, strain gage systems installed on individual pipes connected to a casing have limited accuracy and precision. The raw strain gage signals generally require correction factors for: temperature, moisture, pressure changes, non-uniform pipe cross sections, torsion and other factors. Furthermore, strain gages measure the forces imparted by individual pipes on a turbine casing and do not directly measure the deflection of a casing. The deflection of the casing is often due to multiple piping forces that deform the casing in a non-linear fashion. Complicated analyses must be performed to derive the deflection of the turbine casing from the individual strain gage measurements.
There is a long felt need for techniques to measure accurately the deflection of a turbine casing due to forces, moments and torques imparted by interconnecting pipes on the casing. Such a technique is needed to validate that the pipes do not apply excessive forces, moments and torques on the turbine casing, during turbine assembly as well as startup and shutdown transients. Such a technique is also needed to identify the occurrence of excessive piping forces. Corrective action may be taken once these excessive piping forces are identified. The desired technique should detect when excessive piping forces and moments of torque are being applied to the turbine casing which may result in a loss of radial clearance control between the rotating parts and stationary seals in a turbine.